ORIGINAL SCIENTIFIC PAPER
Croat. Chem. Acta 2015, 88(4), 505–510
Published online: March 5, 2016
DOI: 10.5562/cca2785
Role of Sulfur as a Reducing Agent for the
Transition Metals Incorporated into
Lithium Silicate Glass
M. Y. Hassaan,1,* M. M. El-Desoky,2 M. G. Moustafa,1 Y. Iida,3 S. Kubuki,3 T. Nishida4
1
Physics Department, Faculty of Science, Al-Azhar University, Nasr City, 11884, Cairo, Egypt
2
Physics Department, Faculty of Science, Suez University, Suez, Egypt
3
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachi-Oji, Tokyo 192-0397, Japan
4
Department of Biological and Environmental Chemistry, Faculty of Human-Oriented Science and Engineering, Kinki University, Iizuka, Fukuoka 820-8555, Japan
* Corresponding author’s e-mail address: [email protected]
RECEIVED: November 11, 2015

REVISED: December 26, 2015

ACCEPTED: December 28, 2015
T HIS PAPER IS DEDICATED TO DR . SVETOZAR M USIĆ ON THE OCCASION OF HIS 70 TH BIRTHDAY
Abstract: Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 glass was prepared with and without 5 wt % sulfur (S) while melting the mixture of the starting materials at 1350 C for 1 h in air. A part of the as-prepared glass was heat treated for 1 h near its crystallization temperature (Tc) as determined
from differential thermal analysis. Each glass was also investigated by means of Mössbauer spectroscopy, X-ray diffraction, FTIR, and DC
conductivity. The Mössbauer spectra showed ionic Fe2+ and Fe3+ species in the glass as well as in the precipitated phase obtained after heat
treatment. XRD patterns demonstrated the glassy phase formation in the as-quenched samples irrespective of the presence of sulfur. The heat
treated samples showed different precipitated phases containing iron particles of nanometer size. The electric conductivity measurements
showed that sulfur-doped samples had high values of () probably because of small polaron hopping between Fe2+ and Fe3+.
Keywords: Mössbauer spectroscopy, electrical conductivity, sulfur, heat treatment.
INTRODUCTION
L
ITHIUM silicate glass and related ceramic have
important applications such as heat-resistant glasses,
glass fiber, optical glass, bioglass and bioglass-ceramic.[1–2]
Glass-ceramics are produced by controlled heat treatment
of glass at temperatures near its Tc. If the heat treatment
time is short (e.g., less than 1 h), the size of crystalline
particles is expected to be of nanometer size, and provides
interesting physical properties.[3–4] Transition metal (TM)
ions introduced in the glass matrix improve the
physicochemical properties of glass and glass-ceramics.
These ions improve the electrical conduction due to
electron hopping from the lower valence state to the higher
state (e.g. from Fe2+ to Fe3+).[5]
Recently, much attention has been paid to the
development of fast-ion conductors of glass and glassceramics. Li-ion batteries (LIB) are the most popular source
for different modern portable electronic devices.[6–8]
Lithium silicate glass was found to be usable as fast-ionic
conductor even at low temperatures, because of its high
mechanical strength and chemical stability.[9] It could be used
for ceramic-metal sealing and for dental restoration.[10–11]
In the present study, we investigated the dc
conductivity of Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 glass and its
glass-ceramics, in addition to Mössbauer, DTA, IR, density
and XRD measurements. Since sulfur is a reducing agent, we
added it to the reagent mixture to control the ionic states
of Ni and Fe to facilitate the electronic conduction beside
the ionic one in the glass and glass-ceramic samples.
EXPERIMENTAL
Glass samples of the composition Li2O·0.25Fe2O3·0.25NiO·
1.5SiO2 (in mol %) with and without 5 wt % of sulfur (S) were
prepared by melt-quenching method in a platinum crucible
506
M. Y. HASSAAN et al.: Role of Sulfur as a Reducing Agent …
at 1350 C for 1 h in air using an electric furnace. The melt
was quenched on a copper plate and yielded a glass plate
of 1 mm thickness. Heat treatment was conducted for 1 h
at 550C, being close to its crystallization temperature (Tc)
as obtained from DTA (Shimadzu). The bulk densities were
measured at room temperature using Archimedes method
with carbon tetrachloride as the immersing liquid (r = 1.593
g cm–3). XRD measurements of glass and heat-treated
samples were conducted using [Rigaku RINT 2100] with
CuKα (0.1541 nm) radiation. The maximum current and
voltage were 300 mA and 50 kV, respectively. FT-IR spectra
were recorded at room temperature by standard KBr pellet
method using computerized FT-IR spectrophotometer
[JASCO FT-IR-300E] in the range (400–4000 cm–1).
The DC electrical conductivity for glass and glassceramics were measured by means of two-probe method,
which is appropriate for high resistance materials. Polished
surface of each sample was coated with silver paste, and
two polished cleaned copper electrodes was attached to
the coated area. A Pico-ammeter 760 was used to collect
the DC data over the temperature range of 300–550 K. The
sample temperature was measured by a chromel–alumel
type thermocouple which was placed at a position to the
sample as close as possible. The Mössbauer spectra were
recorded at room temperature by a constant acceleration
method with Wissel MDU-1200, connected to a digital
function generator (Wissel DFG-1000). A source of 57Co(Rh)
of 925 MBq and α-Fe foil were used as a source and a
reference, respectively. Mössbauer spectra were analyzed
assuming a Lorentzian peak shape using the fitting program
Mösswinn 3.0i XP.
RESULTS AND DISCUSSION
DTA
DTA was used to determine the characteristic
temperatures of materials. Fig. 1 shows the DTA curve of
the as-quenched glass with a composition of
Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 containing 5 wt % of
sulfur. Glass formation was confirmed from the glass
transition temperature (Tg) observed at 458 °C, and
successive three exothermic peaks corresponding to
crystallization temperatures (Tc1, Tc2, Tc3) observed at 597,
664 and 736 °C, respectively, followed by endothermic
effect due to melting (Tm) at 1015 °C.
Density
Densities of the as-quenched glass sample with and without
5 wt % sulfur are listed in Table 1, together with those for
HT samples. Table 1 show that the density of S-free sample
is greater than that of S-doped sample. The decrease in the
density after sulfur addition indicates the reduced rigidity
Croat. Chem. Acta 2015, 88(4), 505–510
Figure 1. DTA curve of as-quenched glass with a composition
Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 plus 5 wt % sulfur.
of the glass matrix; there is a weakening of the structure,
which will be favorable for the higher mobility of cations
and consequently higher electrical conductivity (see section
“Electrical Conductivity”). On the other hand, density of HT
samples was greater than that of corresponding asquenched glasses. The increase in the density of HT samples
may be due to the rearrangement of ions causing structural
changes to form glass-ceramics, i.e. the ions closely packed
than in glass.[4]
X-ray Diffraction (XRD)
XRD of the glass samples and its HT samples was
measured using CuKα radiation at room temperature to
examine their amorphous and/or crystalline nature. Fig. 2
shows XRD pattern of Li2O·0.25Fe2O3·0.25NiO·1.5SiO2
glass with 5 wt % sulfur (bottom), and those of HT samples
with and without sulfur. In the case of as-quenched
glasses, as shown in Fig. 2 (bottom), a representative hallo
was observed at around 2θ = 20–35 degrees with no
diffraction peaks, confirming the glass formation. In the
case HT-samples, some sharp peaks were observed being
ascribed to LiFeSi2O6 (JCPDS card 71–1064) with a
structure close to monoclinic, Fe 21.32O34 (JCPDS card 80–
2186) with a structure close to tetragonal and Fe 2O3
(JCPDS card 89–0596) with a structure close to
rhombohedral.
Table 1. Densities of Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 glasses
with and without sulfur (S) and those of corresponding HT
samples.
Samples with S /
wt %
Density of glass
samples / g cm–3
Density of HT
samples / g cm–3
0
2.86
2.89
5
2.76
2.81
DOI: 10.5562/cca2785
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M. Y. HASSAAN et al.: Role of Sulfur as a Reducing Agent …
Using XRD data, the average crystallite size was
calculated from Williamson–Hall (W-H) plot between βcosθ
and sinθ, as given in Fig. 3.[4,12] The Williamson-Hall
equation is
β cos θ 
cλ
 4ε sin θ ,
Dave
(1)
where β is the fullwidth at half maximum (FWHM), θ is the
Bragg angle, c the correction factor (c ≈ 1), Dave the average
crystallite size, λ the wavelength of X-ray, ε the lattice
strain. It is observed that the average crystallite size of Sfree sample is greater than that of S-doped one.
Fourier-Transform Infrared Spectroscopy
Figure 2. XRD patterns of Li2O·0.25Fe2O3·0.25NiO·1.5SiO2
glasses with 5 wt % sulfur (S) and HT samples with and
without S.
Figure 3. W-H plot for Li2O·0.25Fe2O3·0.25NiO·1.5SiO2
samples (HT) with and without 5 wt % sulfur (S).
DOI: 10.5562/cca2785
FTIR spectroscopy is an important tool for the characterization of glass sample. Figure 4 shows FTIR absorption
spectra of HT samples of S-doped and S-free glasses. In the
case of S-free glass, the broad band observed at ~ 485 cm–1
was assigned to Si-O-Si bending vibration of SiO4
tetrahedra.[13–15] After HT, this broad band slightly shifted
to a high wavenumber region of 493 cm–1. In the case of Sdoped glass, the broad band observed at ~ 478 cm–1 was
assigned to Si–O–Si bending vibration in SiO4 tetrahedra.
This broad band shifted to 485 cm–1 after HT.
Figure 4. FTIR absorption spectra of Li2O·0.25Fe2O3·
0.25NiO·1.5SiO2 glasses with and without 5 wt % sulfur (S),
together with the spectrum of HT sample (S-free).
Croat. Chem. Acta 2015, 88(4), 505–510
508
M. Y. HASSAAN et al.: Role of Sulfur as a Reducing Agent …
In the case of S-free glass, a weak shoulder at
732 cm–1, attributed to the symmetric stretching vibration
of Si–O–Si bridging bonds, was observed at the same
position after HT.[16] After the sulfur addition, this shoulder
was observed at almost the same position. The bands
appearing in the region of 933–995 cm–1 were attributed to
the asymmetric stretching vibrations of Si-O-Si bridging
bonds.[16] The band observed at 1427 cm–1 was assigned to
the deformation vibration of H–O–H groups in the structure
of glass or surface adsorbed water.[17] The band appearing
at 1596 cm–1 will be due to the deformation vibration mode
of the H2O.[4,18] The band observed at around 3425 cm–1 is
ascribed to the stretching vibration of OH group.[4,18]
Mössbauer Spectroscopy
Figure 5 shows Mössbauer spectra measured at room
temperature. In the case of S-free samples, Mössbauer
spectra exhibited one doublet of Fe3+ tetrahedral (Td) in
both the as-quenched and HT samples. In case of S-doped
glass, the above doublet appeared with another weak
doublet due to Fe2+ (Oh). In the sample doped sulfur, peaks
due to Fe2O3 precipitated observed as a result of heat
treatment, as was confirmed the XRD study. Mössbauer
parameters are summarized in Table 2.
Electrical Conductivity
Figure 6 shows a semiconducting temperature dependence
of the electrical conductivity σ(T), for S-free and S-doped
glasses and that for HT samples, which are described by
Mott equation (2).[5,19]
σT  σ0 exp(W / kT )
(2)
where o is a pre-exponential factor as discussed below.
The activation energy (W) and pre-exponential factor (σ0)
were obtained from the least square fits of the data. The
straight line nature of the Mott-type plot indicates that the
conduction is thermally activated, as often found in several
semiconductors. Value of W reflecting the thermal energy
required to promote the polaron hopping from one site to
another was calculated to be 0.51–0.59 eV. Values of  and
W obtained at 408 K are listed in Table 3.
Figure 5. RT Mössbauer spectra of Li2O·0.25Fe2O3·0.25NiO·
1.5SiO2 glasses with and without 5 wt % sulfur (S), and those
of HT samples.
In Fig. 6 and Table 3 are shown dc conductivities of Sdoped and S-free glasses, together with those for HT samples measured at a fixed temperature (408 K). They indicate
that the conductivity of S-doped glass samples is higher
than that of S-free glass. This may be due to the presence
of Fe2+ ions in S- doped glass and its HT sample, as
confirmed by the Mössbauer study. It is considered that
sulfur atoms acted as a reducing agent in S-doped glass,
which caused the reduction of Fe3+ to Fe2+ ions.[20] When
the glass was heat treated at 550 °C for 1 h, randomly
dispersed nanocrystals were formed in the glass matrix. As
a result, the electrical conductivity was one order of
Table 2. RT-Mössbauer parameters of Li2O·0.25Fe2O3·0.25NiO·1.5SiO2 glasses with and without 5 wt % sulfur (S) and those for
HT samples.
Samples
Fe3+(Td)
Fe2+(Oh)
IS / mm s–1
QS / mm s–1
LW / mm s–1
A/%
IS / mm s–1
QS / mm s–1
LW / mm s–1
A/%
S-free Glass
0.30
0.97
0.56
100
–
–
–
–
S-free Glass (HT)
0.31
0.96
0.59
100
–
–
–
–
5 wt % S- Glass
0.30
0.96
0.56
85.7
1.08
2.066
0.42
14.3
5 wt % S-Glass (HT)
0.28
1.01
0.56
86.0
1.10
2.20
0.42
14.0
Croat. Chem. Acta 2015, 88(4), 505–510
DOI: 10.5562/cca2785
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M. Y. HASSAAN et al.: Role of Sulfur as a Reducing Agent …
In the range of measurements (Table 3), W depends
on the site-to-site distance R. These results show that there
is a correlation between W and R between iron ions. The
effect dependence of  on the present samples can be
explained by the changes of the distance between the iron
ions, R. It is concluded, therefore, that the conduction
between iron ions depends strongly on R which is important
to the conduction mechanism of our samples. This agrees
with the results obtained recently by El-Desoky.[22]
CONCLUSION
Figure 6. Temperature dependence of dc conductivity (σ T)
for the glasses with and without S and their (HT) samples.
magnitude smaller than that of as-quenched glass. It is
speculated that nanocrystals produce ion-blocking effect
imposed in the lithium ions in the glass matrix. This blocking
effect was discussed for the oxygen-conducting nanomaterials.[21] In our samples, the blocking effect on the oxygen
ions might be caused by grain boundaries or pores that
formed in the samples after heat treatment.
The concentration of iron ions N (cm–3) may be
evaluated using the density by the relation:
XRD patterns confirmed the glassy and precipitated nature
of as-quenched glass samples before and after heat
treatment, respectively. Particle size of the precipitated
nanocrystals was estimated after the HT process. Density of
sulfur-free glass sample was higher than that of sulfurdoped sample. Decrease of the density in the sulfurcontaining glass sample was consistent with the reduced
rigidity. Mössbauer spectra of S-free samples showed one
doublet due to tetrahedral Fe3+ in both the as-quenched
and HT samples. In S-doped samples, a weak doublet due
to Fe2+ appeared along with the doublet of Fe3+ proving that
sulfur acted as a reducing agent for Fe3+. Glass sample
doped with 5 wt % sulfur exhibited higher conductivity and
smaller activation energy than S-free samples. This is due to
the presence of Fe2+ ions in addition to Fe3+ ions. HTsamples showed lower conductivity due to the blocking
effect.
(3)
N  dpNA / AW
where d is the density of the sample, as evaluated above
and presented in Table 1, p is the weight percentage of
atoms, NA is the Avogadro's number and AW the molecular
weight. While the relationship between N and the mean
distance R between iron ions is generally described as:
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DOI: 10.5562/cca2785